"The beautiful user interface that comes out of the box is great as people are able to pick up intuitively. The ease of configuring all modules to suit our specific requirements is commendable. Frappe framework that powers ERPNext has also been utilised a lot by us to plug in various middlewares into ERPNext as well as build stand alone applications. For us, Frappe is the best thing about ERPNext."

What happens after my enterprise contract expires?

You can continue to use ERPNext on your server after your enterprise support license expires, but you will have to manage your own support and server management. Some features like in-app support will also stop working.

If ERPNext is Free and Open Source, why charge per user?

ERPNext is Free and Open Source ("free" as in "free-speech"), but the services around ERPNext need engineers to support you. These are what is charged.
Since scale, complexity and support requirement of a deployment is proportional to the number of users, the Enterprise Support contract is also priced per user.
Also most ERP products are priced per user, so it is easy to compare with other products. ERPNext Support is a fraction of what you would pay for NetSuite, SAP or Oracle Cloud.

How is the number of users counted?

The number of users at the time of signup is counted for the Support Billing. If the number of users exceeds by 15% of the billable users, additional billing is required, for continued support.